G Protein Properties: Identifying the INCORRECT Statement

The question asks to identify the incorrect statement regarding G proteins. Let's analyze each statement:

• Statement 1: GDP is bound to G protein in the resting stage
  This statement is correct. In its inactive or resting state, the G protein is bound to Guanosine Diphosphate (GDP).
• Statement 2: GTP bound $\alpha$ subunit cannot reassemble with $\beta\gamma$ dimer
  This statement is correct. When the G protein is activated, the $\alpha$ subunit exchanges GDP for Guanosine Triphosphate (GTP). The GTP-bound $\alpha$ subunit dissociates from the $\beta\gamma$ dimer and interacts with a target protein. The $\alpha$ subunit only reassembles with the $\beta\gamma$ dimer after GTP hydrolysis back to GDP.
• Statement 3: All G proteins are trimeric
  This statement is incorrect. While many G proteins involved in signal transduction, such as those coupled to G protein-coupled receptors (GPCRs), are heterotrimeric (composed of $\alpha$, $\beta$, and $\gamma$ subunits), there are also monomeric G proteins (e.g., Ras superfamily).
• Statement 4: Activation of G protein may result in activation or inhibition of the target enzymes
  This statement is correct. Depending on the specific G protein and the downstream target, G protein activation can lead to either the stimulation (e.g., Gs activating adenylyl cyclase) or inhibition (e.g., Gi inhibiting adenylyl cyclase) of target enzymes or signaling pathways.

Conclusion

Based on the analysis, the statement "All G proteins are trimeric" is the only incorrect statement among the options provided. Therefore, option C is the correct answer.